An optical amplifier apparatus used at a receiver or a repeater in a wavelength-division multiplexed optical transmission system includes an optical fiber amplifier that uses a fiber doped with a rare earth element, generally, an erbium-doped fiber (hereinafter abbreviated “EDF”). FIG. 1 is a block diagram of a prior art optical fiber amplifier.
The optical fiber amplifier 1 includes the following components between an input end Ti and an output end To: a first optical amplification unit 10 for amplifying input light entering from the input end Ti, a variable optical attenuation unit 20 for attenuating the light amplified by the first optical amplification unit 10, and a second optical amplification unit 30 for amplifying the light passed through the variable optical attenuation unit 20. An output port 21 and an input port 22, between which is inserted a dispersion-compensating fiber (DCF) 22 for compensating for the dispersion of light in a transmission line, are provided between the variable optical attenuation unit 20 and the second optical amplification unit 30.
The light input to the first optical amplification unit 10 is passed through an optical splitting coupler 11 and an optical wavelength-division multiplexing coupler 12 in this order, and then supplied into an EDF 13. Pump light from a laser diode (LD) 14 is also supplied via the optical wavelength-division multiplexing coupler 12 into the EDF 13 where the input light is amplified by the gain proportional to the power of the pump light. The amplified light is passed through an optical splitting coupler 15 and emerges from the first optical amplification unit 10.
The optical fiber amplifier 1 is provided with photodetectors (PDs) 41 and 42, which respectively detect the light levels of the input light to and output light from the first optical amplification unit 10. The photodetector 41 converts the light separated by the optical splitting coupler 11 into an electrical signal. The photodetector 42 converts the light separated by the optical splitting coupler 15 into an electrical signal. The electrical signals from the photodetectors 41 and 42 are supplied to an automatic gain control (AGC) unit 16.
A control signal from the automatic gain control unit 16 is supplied to the laser diode 14, a pump signal generators and the automatic gain control unit 16 controls the signal gain of the first optical amplification unit 10 to a constant level. The gain versus wavelength characteristic of the EDF 13 is thus maintained constant.
The light amplified by the first optical amplification unit 10 is attenuated by the variable optical attenuation unit 20, and then passed through an optical splitting coupler 52 and supplied via the output port 21 into the dispersion-compensating fiber 22. As the light passes through the dispersion-compensating fiber 22, the dispersion that the light suffers when transmitted through the transmission line is compensated for, and the dispersion compensated light is again input to the optical fiber amplifier 1 via the input port 23 and fed into the second optical amplification unit 30.
The light input to the second optical amplification unit 30 is passed through an optical splitting coupler 31 and an optical wavelength-division multiplexing coupler 32 in this order, and then supplied into an EDF 33. Pump light from a laser diode 34 is also supplied via the optical wavelength-division multiplexing coupler 32 into the EDF 33 where the input light is amplified by the gain proportional to the power of the pump light.
The optical fiber amplifier 1 is provided with photodetectors (PDs) 44 and 45 which respectively detect the light levels of the input light to and output light from the second optical amplification unit 30. The photodetector 44 converts the light separated by the optical splitting coupler 31 into an electrical signal. The photodetector 45 converts the light separated by an optical splitting coupler 35 into an electrical signal. The electrical signals from the photodetectors 44 and 45 are supplied to an automatic gain control unit 36.
A control signal from the automatic gain control unit 36 is supplied to the laser diode 34, a pump light emitter, and the automatic gain control unit 36 controls the signal gain of the second optical amplification unit 30 to a constant level. The light amplified by the second optical amplification unit 30 is passed through the optical splitting coupler 35 and emerges from the second optical amplification unit 30; the light is further passed through the output end To and output from the optical fiber amplifier 1.
The optical fiber amplifier 1 is also provided with a DCF loss compensating unit 51 for compensating for variations in the amount of loss in the dispersion-compensating fiber 22 installed between the output port 21 and the input port 23. The DCF loss compensating unit 51 receives the electrical signals from the photodetectors 42 and 44, and controls the amount of loss (attenuation) in the variable optical attenuation unit 20 so that the difference between the output light level of the first optical amplification unit 10 and the output light level of the dispersion compensating fiber 22 is held constant, thus maintaining the total amount of loss, i.e., the sum of the amount of loss in the variable optical attenuation unit 20 and the amount of loss in the dispersion-compensating fiber 22, at a constant level by compensating for variations in the optical loss of the dispersion-compensating fiber 22.
The optical fiber amplifier 1 further includes a DCF disconnection detecting unit 62 which outputs a DCF-disconnected notification when it is detected that the dispersion-compensating fiber 22 is disconnected from the output port 21 or the input port 23, and an abnormal loss detecting unit 61 which outputs an abnormal loss notification when it is detected that the amount of loss in the dispersion-compensating fiber 22 is excessive. The DCF-disconnected notification is delivered, for example, to another optical amplifier provided in the upstream direction, and the upstream optical amplifier that received the DCF-disconnected notification recognizes the occurrence of abnormality in the optical fiber amplifier 1 and performs processing for safety, for example, by reducing the signal output level.
Alternatively, the DCF-disconnected notification is delivered to the automatic gain control unit 16, and the automatic gain control unit 16 that detected the occurrence of abnormality in the optical fiber amplifier 1 performs processing for safety, for example, by reducing the gain of the first optical amplification unit 10.
The abnormal loss notification is used to provide warning to the user after starting the operation. Abnormality in the optical loss of the dispersion-compensating fiber 22 could lead to signal degradation.
FIG. 2 is a level diagram of the light levels detected within the optical fiber amplifier 1 for illustrating a first example of a prior art abnormality detection method. The diagram illustrates the light levels detected by the photodetectors 41 to 45 when the number of multiplexed wavelengths input to the optical fiber amplifier 1 is 0, 1, and 40, respectively. The photodetector 43 converts the light separated by the optical splitting coupler 52, provided between the variable optical attenuation unit 20 and the dispersion-compensating fiber 22, into an electrical signal.
In the diagram, solid lines between the PDs 42 and 44 indicate the light levels detected when the dispersion-compensating fiber 22 having a maximum amount of loss permitted by the rating of the optical fiber amplifier 1 as the amount of loss due to the insertion of the dispersion-compensating fiber is installed between the output port 21 and the input port 23, and semi-dashed lines indicate the light levels detected when the dispersion-compensating fiber 22 having a minimum amount of loss permitted by the rating of the optical fiber amplifier 1 is installed between the output port 21 and the input port 23.
Hereinafter, the maximum and minimum values permitted by the rating of the optical fiber amplifier 1 for the amount of loss in the dispersion-compensating fiber 22 installed between the output port 21 and the input port 23 are designated Lomax and Lomin, respectively.
Further, the amount of loss in the variable optical attenuation unit 20 that is necessary to provide compensation when the amount of loss in the dispersion-compensating fiber 22 is Lomax, i.e., the target value to which the amount of loss in the variable optical attenuation unit 20 is to be controlled by the DCF loss compensating unit 51 when the amount of loss in the dispersion-compensating fiber 22 is Lomax, is designated Vo1, and the amount of loss in the variable optical attenuation unit 20 that is necessary to provide compensation when the amount of loss in the dispersion-compensating fiber 22 is Lomin is designated Vo2. The DCF loss compensating unit 51 controls the amount of loss in the variable optical attenuation unit 20 so that the sum of the amount of loss in the variable optical attenuation unit 20 and the amount of loss in the dispersion-compensating fiber 22 becomes equal to a constant value given by Vo1+Lomax (=Vo2+Lomin).
When the output light level of the dispersion-compensating fiber 22, detected by the photodetector 44, is smaller than a predetermined threshold value T1, the DCF disconnection detecting unit 62 outputs the DCF-disconnected notification by determining that the dispersion-compensating fiber 22 is disconnected. The threshold value T1 is set to a value that is lower by a predetermined margin than the minimum value Lv4 that the light level detected by the photodetector 44 can take when the optical fiber amplifier 1 is operating normally.
The minimum value Lv4 is determined in advance by assuming the situation in which the number of multiplexed wavelengths input is 0 and the amount of loss in the variable optical attenuation unit 20 remains at Vo2 though the dispersion-compensating fiber 22 whose amount of loss is Lomax has been installed. The light level in this situation is indicated by a two-dot dashed line.
When the difference between the light levels before and after the dispersion-compensating fiber 22, i.e., the difference between the electrical signals output from the photodetectors 43 and 44, is larger than a predetermined threshold value TL, the abnormal loss detecting unit 61 outputs the abnormal loss notification by determining that the loss in the dispersion-compensating fiber 22 is excessive. The threshold value TL is set equal to a value obtained by adding a margin to the maximum value Lomax permitted by the rating of the optical fiber amplifier 1 for the amount of loss in the dispersion-compensating fiber.
Patent document 1 below discloses a method in which amplified spontaneous emission (ASE) light is transmitted from the upstream amplifier, and using this light, the transmission line loss is measured and the gain of the amplifier is determined before starting the operation.
Patent document 1: Japanese Unexamined Patent Publication No. 2004-072062
Patent document 2: Japanese Unexamined Patent Publication No. 2006-121110
Patent document 3: Japanese Unexamined Patent Publication No. H10-51057